264 EVOLUTION OF PLANTS 
which bring them near the surface of the water where 
they may be exposed to the light. 
One of the most important differences between fresh- 
water and marine alge, resulting from the nature of 
their environment, is the different character of some of 
the reproductive parts. Owing to the constant level 
of.the ocean, aside from the periodic fluctuation of the 
tides, marine plants are never exposed to the complete 
desiccation to which nearly all fresh-water plants are 
at times liable; nor is there nearly so much difference 
of temperature in the water at different seasons, as in 
the shallower and usually variable body of water in 
most lakes and rivers. We find, therefore, that the 
marine alge do not develop resting-spores except in 
rare instances, but the spores are thin-walled or naked 
cells which germinate as soon as they are mature. 
Where the plants show a definite periodicity in their 
growth, as not infrequently occurs in the colder 
northern waters, the plant is usually perennial by 
means of a sort of root-stock, or rhizome, from which 
the annual shoots are produced. 
Most fresh-water alge, however, are plants of very 
limited growth, and are usually destroyed either by 
freezing or drying up at the end of their growing 
season. In the great majority of these are produced 
special reproductive bodies, usually resting-spores, 
which are capable of resisting extremes of tempera- 
ture and dryness which would quickly destroy the 
actively vegetating plant. These resting-spores are 
commonly the result of fertilization, but not infre- 
quently they may form non-sexually, as we find in 
various of the fission alge, like Nostoc or Anabena. 
